The compound 4-(4-hydroxyphenyl)butan-2-one also known as Raspberry ketone exhibits its characteristic raspberry odour and is used as an important aroma chemicals in perfume industries, food industries and also in compositions for weight loss with improved taste. Raspberry ketone was described for the first time as a characteristic component of Raspberry flavour (H. Schinz et. al. Helv. Chim. Acta. 1957, 40, 1839). However, this substance was already known in its natural state as a component of a polygonacaea. The proportion of raspberry ketone in the raspberry is only between 9 and 174 μg per kilogram of fresh fruit depending on the variety (W. Borejsza-Wysocki et. al. J. Food Chem., 1992, 40, 1176). Thus, the concentration of raspberry ketone in raspberry fruit is so low that extraction of the compound from the fruit would not be economically viable. Therefore, there is no practical natural source for isolation of such important aroma chemical.
Reference may be made to K. Krishnan et. al. Appl. Cata. A: General, 241 (2003) 247-260, wherein liquid phase alkylation of phenol (about 5 mole) with 4-hydroxybutan-2-one (about 1 mole) was carried out in presence of different types modified zeolite, H-beta zeolites and other solid catalysts for a period of 48-60 hours at 100° C. to produce raspberry ketone. The main drawback of the process is the selectivity which is less than 40%.
Reference may be made to D. Joulain et. al. U.S. Pat. No. 5,908,770, June 1999 wherein a microbiological preparation of 4-(4-hydroxyphenyl)butan-2-one has been described where a solution of 4-(4-hydroxyphenyl)but-3-en-2-one in ethanol at 96° C. was added very slowly over 10 minutes to a suspension of baking yeast in D-glucose at 35° C. and agitated the mixture for about 48 hours followed by continuous liquid-liquid extraction for about 12 hours using methyl-tertiary-butyl ether. After evaporating the liquid, a solid mass of crude product is obtained consisting mainly (GC-MS) about 56% of 4-(4-hydroxyphenyl)butan-2-one and 37% 4-(4-hydroxyphenyl)butan-2-ol. The product was then purified. The main drawback is that the process is time consuming and the selectivity is about 56% only.
Reference may be made to B. K. Osjek et. al. Tetrahedron, 59 (2003) 9517-9521 wherein efficient production of raspberry ketone from rhododendrol rac-2 (rhododendrol is produced in the inner bark of the stressed and drying branches of most white barked birch species by natural hydrolysis of glucoside rhododendrin) via “Green” biocatalyst oxidation has been described. The main drawback of the process is that the conversion is only 52% in 24 hours.
Friedel Crafts catalysis is one of the major industrially important processes that is widely used in the synthesis of low and high volume chemicals through acylations, benzylations, alkylations and sulphonylations giving a wide range of useful products like ketones, alcohols, alkyl aromatics and sulphones. Reference may be made to A. G. Badische et. al. Patent (German) 2145308 wherein raspberry ketone is commercially produced by alkylation of phenol with vinyl ketone or 4-hydroxybutan-2-one in presence of acid catalysts such as H2SO4, H3PO4 and HCl. Main drawback is that the catalysts are hazardous in nature and require replacement in view of environment pollution. The use of safe solid acids in the place of traditional Friedel Crafts catalysts and mineral acids have become important. Ecofriendly solid acid catalysts are rapidly emerging as a new and environment friendly materials. Attempts are being made to replace highly corrosive concentrated HCl, H2SO4, anhydrous AlCl3 in F.C. alkylation reactions etc. by solid acid catalysts. All the solid acids are characterized by the presence of protons on the surface leading to Bronsted and/or Lewis acidity.
Reference may be made to P. Laszlo and A. Mathy; Helvetica Chimica Acta 70 (1987) 577 wherein Friedel-Crafts alkylation by using transition metal exchanged Montmorillonite K 10 was carried out for reactions like benzylation of benzene etc. No mention was made for preparation of 4-(4-hydroxyphenyl)butan-2-one.
Reference may be made to J. Org. Chem. 59, 1994, 5901, wherein Cation-exchanged Montmorillonite catalysed alkylation of phenol with 4-hydroxybutan-2-one to produce raspberry ketone was carried out at 100° C. for about 48 hours to produce 4-(4-hydroxyphenyl)butan-2-one. This paper also described several cation exchanged Montmorillonite clay catalysts. The main drawback of the process is that the selectivity (GC analysis) of the desired products is from 3 to 35%. The best catalyst (i.e., Fe3+− Mont.) under the reaction conditions of temperature: 130±50° C.; pressure: 2±1 bar for a time duration of 24 hrs shows selectivity of only 47.54%.